Red-emitting phosphors based on complex fluoride materials activated by Mn4+ can be utilized in combination with yellow/green emitting phosphors such as YAG:Ce or other garnet compositions to achieve warm white light (CCTs<5000 K on the blackbody locus, color rendering index (CRI)>80) from a blue LED, equivalent to that produced by current fluorescent, incandescent and halogen lamps. These materials absorb blue light strongly and efficiently emit between about 610-635 nm with little deep red/NIR emission. Therefore, luminous efficacy is maximized compared to red phosphors that have significant emission in the deeper red where eye sensitivity is poor. Quantum efficiency can exceed 85% under blue (440-460 nm) excitation.
While the efficacy and CRI of lighting systems using Mn4+ doped fluoride hosts can be quite high, many of these materials exhibit some instability in high temperature, high humidity environments and this may limit their use in commercial systems requiring long term stability under operating conditions. U.S. Pat. No. 8,252,613 describes a process that can reduce this degradation by post-synthesis treatment with an aqueous hydrofluoric acid (HF) solution of a hexafluorosilicate salt. However, toxicity of HF is a significant consideration, and alternatives that can maintain the improvement in stability of the materials while reducing the amount or concentration of HF in the treatment solution are desirable.